Tests determining auditory thresholds in human subjects are well known in the art. Typically these tests include the presentation to a subject of a signal, typically a pure tone, over a range of signal intensities and recording the lowest intensity at which the subject reports that he hears the signal. This report typically includes a voluntary response such as pressing a button or raising a hand.
Also known in the art are tests determining auditory thresholds for intelligible speech. These tests determine the minimum intensity at which an auditory stimulus, consisting of spoken meaningful words, must be presented for the words to be heard and understood. Typically, tests determining the auditory threshold of intelligible speech include presenting a stimulus including spoken words to a subject over a range of intensities, having the subject repeat the words he hears, and recording the lowest intensity at which the words are correctly repeated.
It is well established that electrical patterns of the human brain, as recorded in an electroencephalogram (EEG), can reflect cognitive processes. For example, Kutas, M. and Hillyard, S. A. (1988, Contextual effects in language comprehension Studies using event-related brain potentials. In Plum, F. ed., Language, communication and the brain. Raven Press, New York) have studied event related potentials (ERP's) in the EEG pattern in relation to contextual effects in language comprehension. These authors have reported that when human subjects are presented with a sentence on a CRT display and the last word of the sentence is incongruent (i.e. does not make sense in the context of the sentence) the subjects show a specific ERP. This ERP, which is called N400, is a slow negative potential peaking at about 400 msec from the presentation of the last word. This N400 response is not present in subjects presented with a sentence that ends in a congruent word (i.e. a word that makes sense in the context of the sentence).
Similar results have been reported by McCallum et al. (1984, The effect of physical and semantic incongruities on auditory event related potentials. Electroencephalogr. Clin. Neurophysiol., 59, 477-488) with respect to auditory stimuli. These results suggest that N400 ERP's are the subjects' response to incongruity in language, regardless of whether the language is written or spoken.